ØRSTED'S
NONDISCOVERY OF ELECTROMAGNETIC INDUCTION
Note: The Danish/Norwegian letter Ø,
when properly pronounced sounds a little like the "oh" in neighbor.
In the Summer of 1820 Hans Christian
Ørsted, a professor of physics at the University of Copenhagen, made one of the
most spectacular discoveries in the history of Science. In the middle of
teaching a course in physics he performed the first experiment that demonstrated
a relationship between electricity and magnetism. The apparatus of the
experiment consisted of a magnetic compass near a wire and a voltaic pile (a
primitive electric battery) to whose opposite poles Ørsted could connect the
opposite ends of the wire to make an electric current flow in the wire. Ørsted
pointed out to his students that the needle of the compass shifted its
orientation when a current flowed in the wire and that the needle returned to
its original orientation when he broke the circuit and thus made the current
stop flowing. A certain folklore has it that Ørsted made this discovery purely
by accident, but Ørsted's own account (presumably supported by his students)
disputes that interpretation: as preparation for the experiment Ørsted had
mentioned and commented on observations that travelers had made over the years
that their magnetic compasses moved erratically whenever thunderstorms passed
overhead.
Prior to conducting his classroom
experiment, Ørsted had hypothesized that something in thunderstorms exerts a
magnetic force upon travelers' compasses. He knew, as a result of Benjamin
Franklin's famous kite experiment, that thunderstorms contain electricity and
that lightning is a powerful electric current, so he guessed that of all the
phenomena associated with thunderstorms, the electric incandescence of the
lightning deflected the compass needles. He designed his experiment to abstract
that one feature from the thunderstorm and to recreate it in miniature in his
classroom so that he could test his hypothesis in accordance with the standard
Scientific Method. Once he had confirmed his hypothesis he conducted further
experiments to refine it, subsequently discovering that it was not the
incandescence, but rather the electric current alone that exerted the magnetic
force in his experiments.
Though he effectively halted his study of the relationship between electricity and magnetism at that point, he could have gone farther and discovered electromagnetic induction. Aside from a lack of imagination, nothing prevented from taking additional steps. Those steps involved little more than applying certain rules of Newtonian mechanics, which rules he knew as well as did any other physicist of the time. Had he done that, he might have reasoned from his discovery as follows:
1) An electric current (A) exerts a force upon a magnet (B),
2) By Newton's third law of motion, if A exerts a force upon B, then B exerts an equal and oppositely directed force upon A; therefore,
3) A magnet (B) exerts a force upon an
electric current (A).
Prior to taking the next step Ørsted would have had to clarify the concept of an electric current in a way that avoids any confusion in what follows. Because an electric current consists of electric charges in motion, he could have created the concept of a pure electric current by imagining that he had deposited electric charges upon a silk thread and then moved the thread in a direction parallel to its length. He might have imagined, for example, pulling the thread off one spool and winding it onto another one. If the thread were to pass between the poles of a strong magnet, the magnet would, in accordance with Statement #3 above, deflect the thread sideways. Ørsted could then have imagined moving with the thread while a student sat by the magnet. He would then have reasoned:
4) The student (C) sees the thread deflected away from a straight line by the magnet;
5) By the Principle of Relativity, any phenomenon that Reality manifests to Observer C it must also manifest to Observer D; therefore,
6) Ørsted (D) sees the thread
deflected away from a straight line where it passes through the magnet.
We think of Relativity as originating
with Albert Einstein in 1905, but the basic principle appeared in the scientific
literature as long ago as 1633. That's when Galileo Galilei published his
quickly suppressed book, "Dialogue on the Two Chief World Systems". In that book
Galileo described the principle of Relativity (though he didn't call it that) by
noting that there is no experiment one can perform that will reveal one's
uniform motion in a straight line relative to some putative absolute state of
rest; in particular, he noted that a man occupying a windowless cabin aboard a
ship would be unable to make any experiment that would reveal whether the ship
was sailing on a calm sea or was tied to a dock in a port. Isaac Newton offered
a similar description in his Principia. Ørsted certainly knew of it and in this
case might have used it as a version of the law of noncontradiction. Ørsted
would have brought that noncontradiction into play by claiming that uniform
relative motion cannot so deform space and time that a deformation of a thread
into a curve for one observer would be hidden from another observer, who would
see the thread uncurved: the thread must appear curved to both Ørsted and his
student.
But for Ørsted the electrically charged thread would not be moving and, thus, would not constitute an electric current. In his frame the magnet would be moving and the charged thread would be stationary, so he would reason:
7) Only an electric force can move a stationary electric charge;
8) The moving magnet is moving a stationary electric charge; therefore,
9) A moving magnet exerts an electric force.
That last statement gives us the
principle of electromagnetic induction and the scientific foundation upon which
engineers base electric generators. Designed to make wires wound upon rotors
spin within arrays of stationary magnets (only relative motion between the wires
and the magnets counts, after all), electric generators are the equivalent of
voltaic piles, but with the advantage that they can generate electricity from
anything that can spin the rotors (steam engines and falling water are two
common prime movers). Thus, Ørsted could have deduced the principle of
electromagnetic induction in 1820 and laid the foundation for our modern
electrically-driven civilization eleven years earlier than Michael Faraday
actually did.
We call the mathematical expression of
Statement #9 Faraday's law and it is the third of the four fundamental equations
of electromagnetic theory (the equations are called Maxwell's Equations).
Physicists have so named the law because Faraday discovered it in August 1831,
doing so in a way more consistent with the folklore surrounding Ørsted's
discovery; that is, by accident.
Once physicists saw from Ørsted's
experiment that an electric current causes a magnetic effect, they speculated
that some kind of reciprocal relationship must exist between electricity and
magnetism; that is, they speculated that, in some way, a magnet could be made to
cause an electric effect. Faraday hypothesized that he could obtain such an
effect by using the magnetic effect of an electric current in one wire to induce
the desired electric effect in a second wire. In order to test that hypothesis
Faraday wound two wires around an iron core, attached the ends of one wire to a
galvanometer (a device that, by embodying Ørsted's experiment, detects the flow
of electric current with a magnetized needle), and attached the ends of the
other wire to a voltaic pile through a switch. The test immediately falsified
Faraday's hypothesis: the galvanometer showed that no current flowed in the one
wire when current flowed in the other wire. However, Faraday was sufficiently
observant to notice that the needle of the galvanometer twitched whenever he
opened or closed the switch. He inferred then that it was not a steady current,
but rather a changing current, in a wire that induces a current to flow in a
neighboring wire. Applying his own concept of forcefields (developed as an "aid
to the imagination" when he saw that iron filings scattered on paper were
rearranged by the presence of a magnet or of a current-carrying wire), he then
deduced the rule that a changing magnetic field (of which the field of a moving
magnet is an example) generates an electric field, which rule we now know in its
mathematical form as Faraday's law.
We may well ask why it took physicists
eleven years to progress from Ørsted's experiment to the discovery of
electromagnetic induction and had to discover the latter by accident to boot. It
was certainly not for lack of interest. No, I believe that an overemphasis on
empiricism led to an Icarus complex, a dread of allowing the imagination to soar
to high. It took time for physicists to gain confidence in the techniques
implied in the use of Faraday's "aids to the imagination". They had to be shown
the way by outsiders, such as James Clerk Maxwell and Albert Einstein. Today the
use of carefully contrived fantasies is an important part of theoretical physics
and that use has evolved modern physics into a subject that Hans Christian
Ørsted would barely recognize.
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